1. Field of the Invention
The present invention broadly relates to novel ceramic composites and methods of making the same. In a more specific aspect, the invention relates to ceramic composites particularly useful as refractories, such as heat exchanger components and steel plant refractories. The invention also relates to methods of making the ceramic composites by the directed oxidation at elevated temperatures of a parent metal into a permeable mass of filler material followed by a subsequent heating step to remove or oxidize residual non-oxidized metal constituents.
2. Description of Commonly Owned Patents and Patent Applications and Background
The subject matter of this application is related to commonly owned U.S. Pat. No. 4,713,360, which issued on Dec. 15, 1987, and was based on commonly owned U.S. patent application Ser. No. 818,943, filed Jan. 15, 1986, which is a continuation-in-part of Ser. No. 776,964, filed Sep. 17, 1985, which is a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, now abandoned, which is a continuation-in-part of Ser. No. 591,392, filed Mar. 16, 1984, now abandoned, all in the name of Marc S. Newkirk et al. and entitled "Novel Ceramic Materials and Methods for Making the Same". This patent discloses the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent metal precursor. Molten parent metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation reaction product toward the oxidant thereby continuously developing a polycrystalline ceramic body of the oxidation reaction product. The ceramic body can be produced having metallic components and/or porosity, which may or may not be interconnected. The process may be enhanced by the use of an alloyed dopant, such as in the case of an aluminum parent metal oxidized in air. This method was improved by the use of external dopants applied to the surface of the precursor metal as disclosed in commonly owned U.S. patent applications Ser. No. 220,935, filed Jun. 23, 1988, now U.S. Pat. No. 4,853,352, which is a continuation of Ser. No. 822,999, filed Jan. 27, 1986, now abandoned, which is a continuation-in-part of Ser. No. 776,965, filed Sep. 17, 1985, now abandoned, which is a continuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, now abandoned, which is a continuation-in-part of Ser. No. 632,636, filed Jul. 20, 1984, now abandoned, all in the name of Marc S. Newkirk et al., and entitled "Methods of Making Self-Supporting Ceramic Materials".
The subject matter of this application is also related to that of commonly owned U.S. patent applications Ser. No. 819,397, filed Jan. 17, 1986, now U.S. Pat. No. 4,851,375, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, now abandoned, both in the name of Marc S. Newkirk et al. and entitled "Composite Ceramic Articles and Methods of Making the Same". This patent discloses a novel method for producing self-supporting ceramic composites by growing an oxidation reaction product from a parent metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The self-supporting ceramic composite produced thereby is characterized by an essentially single-phase polycrystalline oxidation reaction product and distributed metal or voids or both, and by crystal lattice misalignments at oxidation reaction product crystallite grain boundaries less than the lattice misalignments between those neighboring oxidation reaction product crystallites having planar metal channels or planar voids, or both, disposed between said neighboring crystallites. Further, the interface boundaries between the oxidation reaction product and the metallic phase are substantially arcutate and they weave or form a tortuous microstructure also has been observed when voids were formed in place of metal.
Further developments of the foregoing methods enable the formation of ceramic composite structures which (1) contain therein one or more cavities which inversely replicate the geometry of a shaped precursor parent metal, and (2) have a negative pattern which inversely replicates the positive pattern of a parent metal precursor. These methods are described, respectively, (1) in Commonly Owned U.S. patent application Ser. No. 823,542 filed Jan. 27, 1986, now U.S. Pat. No. 4,828,785, in the name of Marc S. Newkirk et al, entitled "Inverse Shape Replication Method of Making Ceramic Composite Articles and Articles Obtained Thereby", and (2) in Commonly Owned U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, now U.S. Pat. No. 4,859,690, in the name of Marc S. Newkirk, and entitled "Method of Making Ceramic Composite Articles with Shape Replicated Surfaces and Articles Obtained Thereby".
Also, methods of making ceramic composite structures having a preselected shape or geometry were developed. These methods include the utilization of a shaped preform of permeable filler into which the ceramic matrix is grown by oxidation of a parent metal precursor, as described in Commonly Owned U.S. Pat. No. 5,017,526, in the names of Marc S. Newkirk, et al. and entitled "Method of Making Shaped Ceramic Composites", and was based on U.S. patent application Ser. No. 338,471, filed Apr. 14, 1989, which is a continuation. U.S. patent application Ser. No. 861,025, filed May 8, 1986, in the name of Marc S. Newkirk et al. and entitled "Shaped Ceramic Composites and Methods of Making the Same". Another method of making such shaped ceramic composites includes the utilization of barrier means to arrest or inhibit the growth of the oxidation reaction product at a selected boundary to define the shape or geometry of the ceramic composite structure. This technique is described in Commonly Owned U.S. patent application Ser. No. 861,024, filed May 8, 1986, now U.S. Pat. No. 4,923,832, in the name of Marc S. Newkirk et al. and entitled "Method of Making Shaped Ceramic Composites with the Use of a Barrier".
The entire disclosure of all of the foregoing Commonly Owned Pat. Applications and Pat. are expressly incorporated herein by reference.
Common to each of these Commonly Owned Patents Applications and Patent is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product, most typically interconnected in three dimensions, and, optionally, one or more non-oxidized constituents of the parent metal or voids or both. The metal phase and/or the voids may or may not be interconnected depending largely on such factors as the temperature at which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, etc. For example, if the growth process is continued to substantially exhaust (convert) the metal constituents, porosity will result as a partial or nearly complete replacement of the metal phase throughout the bulk of the composite body, while developing a dense ceramic skin at the surface of the composite body. In such a case, the interconnected porosity is typically accessible from the surface of the ceramic body from which matrix development initiated.
Ceramic refractories are useful as components for applications requiring good resistance to thermal shock, corrosion and erosion in contact with molten metals. Such components may, for example, be used in control means for regulating the flow of molten metals in molten metal transfer systems, for example, in the manufacture and handling of steel. Such uses include, for example, slide gates, submerged entry nozzles, and ladle shrouds. Slide gates are used for controlling the flow of molten metal from a ladle. Generally, slide gate systems including some rotary designs, consist of a fixed nozzle attached to and within a movable plate. The flow of molten metal from a ladle is controlled by moving the movable plate to fully or partially align openings. When filling the ladle and during shut-off, the openings are misaligned. The principal advantage of the slide gate system over a conventional stopper rod system is its improved reliability of shutoff, ability to modulate molten metal flow, and lack of aspiration of the molten steel product stream. However, even the best slide gate systems, such as high-alumina slide gate systems, are inadequate for certain molten metals, such as specialty steel like low-carbon, high-manganese grades. These corrosive steel compositions will seriously attack the bonding media used in most high-alumina grade slide gate systems.
Today, in the United States market, the majority of the slide gate refractories are composed of either tar-impregnated high-alumina, or fired magnesia materials. However, such slide gate refractories do not possess the thermal shock, corrosion and erosion resistance criteria to stand up to long ladle holding and teeming times and preheating, and therefore have a short service life.
The ceramic composites of this invention offer potential for use as steel plant refractories such as slide gate refractories, that do not have the foregoing deficiencies while still possessing thermal shock, corrosion and erosion resistance criteria to withstand long ladle holding and teeming times and preheating. In addition, they may be useful for other applications requiring thermal shock resistance and high temperature strength retention.